Backdraft rubber skin mold pattern

ABSTRACT

A composite metal and elastomer skin pattern for forming circular sand molds includes a circular metal sleeve which forms the basic mold portion and an elastomer skin or covering bonded thereto which includes the pattern&#39;&#39;s backdraft portion whereby the resulting pattern can be removed from a cured sand mold without damage thereto due to the elastic deformations of the pattern&#39;&#39;s surfaces and its elastomer backdraft portion.

D United States Patent 1 1 [111 3,738,418 Harris 1 June 12, 1973 BACKDRAFT RUBBER SKIN MOLD 3,552,480 1 1971 Harris 164/425 PATTERN FOREIGN PATENTS OR APPLICATIONS [75] Invento G o g V- Harris, Racine, 1,227,020 2/1960 France 285/111 Assignee'z Caterpillar Tractor Peoria In. 1,474,063 2/1967 France 285/110 [22] Filed: July 26, 1971 Primary ExaminerR. Spencer Annear [21] Appl. No.: 166,201 Attorney-Charles M. Fryer, Amt H. Tjensvold,

Donald C. Feix et al.

[52] US. Cl 164/245, 164/44, 249/183 57 S R CT [51] Int. Cl. B22c 7/00 A com osite metal and lastom r kin H r f0 58 Field of Search 164/44, 245, 249; P e e s Pa 8 n 249/l83 18mm 285/] 104 11 forming circular sand molds includes a circular metal sleeve which forms the basic mold portion and an elas- [56] References Cited tomer skin or covering bonded thereto which includes the pattems backdraft portion whereby the resulting UNITED STATES PATENTS pattern can be removed from a cured sand mold with- 6l,641 1/1867 Taylor 164/245 X out damage thereto due to the elastic deformations of Connolly X the patterns surfaces and its elastomer backdraft p0r 3,090,996 5/1963 Reichenbach et a]. 249/83 x tiom 3,378,888 4/1968 Robertson 18/D1G. 58

8 Claims, 5 Drawing Figures PAmnmw "3338.418

. 3 EJLE E El5 4- ENTOR GEOR V. HARRIS 9% W3 wfaw. +1.

ATTORNEYS BACKDRAFT RUBBER SKIN MOLD PATTERN BACKGROUND OF THE INVENTION Prior art patterns have been in use which employ thick elastomer shells with central inserts which are removable from the elastomer shell prior to the removal of the shell from a cured sand mold. U.S. Letters Patent No. 2,491,146, issued to Zahn and U.S. Letters Patent No. 2,476,726, issued to Haas, are illustrative of these types of mold patterns. Other prior art patterns of a related character are those composed principally of a metal part and a joined smaller elastomer part forming the backdraft portions, such as the type shown in U.S. Pat. No. 3,552,480, issued to me on Jan. 5, 1971 and entitled Pattern for Making a Sand Mold with a Backdraft. This latter patent discloses a composite pattern where the elastomer backdraft portion forms only a very small part of the mold and is composed of an elastomer which is locked with the basic metal part, such as with tongue and groove coupling structures.

Patterns of the latter type perform satisfactorily when relatively small devices are being cast from the resulting sand mold, such as circular seal rings up to six inches in diameter. However, these patterns, along with the shell patterns referred to above, perform less satisfactorily in larger diameter casting, such as seal rings having diameters of ten inches and larger. In particular, problems often develop in the larger sand molds formed with the prior art patterns, due to the increased mold size as the pattern is removed, since the metal surfaces provide no resiliency in the major portion of the mold. Also, the shell patterns are not satisfactory where it is desirable to maintain the ultimate casting in tolerances of i 0.030 inches in order to develop a serviceable casting as the thick rubber shell sections tend to be deformed by the compaction of sand around the pattern and fail to maintain the desired tolerances. As a result considerable machining is normally required on the casting cast from the shell type patterns to bring the final part into the necessary dimensional parameters on large circular devices.

In view of such problems it has often been the practice to employ multi-piece mold structures to cast large seal rings and the like with backdraft portions.

Generally the term backdraft employed herein, refers to articles having hooked contours, overhanging or underlying sections and the like in the mold cavity, which projections prevent a pattern from being removed from the cured sand core without employing multiple parting planes in the mold or alternatively a deformable or melting pattern. Obviously since multiple parting planes often require subsequent machining of the cast part the elimination of such parting planes is highly desirable.

In view thereof it is an object of the present invention to provide a serviceable pattern for forming one-piece sand molds for large circular castings having backdraft portions.

Also a collateral object is to improve the economies in fabricating large one-piece sand molds for large circular castings.

Other objects and advantages will be apparent during the description of this invention and the drawings pertaining thereto.

SUMMARY OF THE INVENTION The above objects, as well as other, can be accomplished with a backdraft rubber skinned mold pattern including a circular metal sleeve forming the substantial portion of the mold pattern with the exception of backdraft portion, which metal sleeve is slightly undersize with reference to the size of the desired resulting mold cavity and an elastomer skin bonded to the surfaces of said metal portion, said elastomer skin including integral backdraft portions of the pattern whereby the resulting pattern can be removed from a cured sand mold without damaging the mold due to elastic deformations of the elastomer skin and backdraft portions of the pattern as it is removed. The elastomer skin thickness, with the exception of the backdraft portions, may vary from 0.010 to 0.050 inch and is preferably formed of heat resistant silicone rubber compounds since many of the sand molds are heat cured with the pattern therein.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a broken-away off-set section through a portion of the pattern attached to a sub-base and a mold table and surrounded by sand which is cured to form the mold cavity;

FIG. 2 is a cross-section through the rigid metal DESCRIPTION OF A PREFERRED EMBODIMENT The complete pattern P is illustrated in a brokenaway section in FIG. 1 and includes three principal components, which are its sub-base 10, its circular metal sleeve member 11 and its elastomer skin 12 having an integral backdraft portions 13. Centrally located on the bottom 14 of the sleeve member is a circular rib 15 that is received in a cooperating groove 16 in the sub-base, after the shaped elastomer skin has been conformed to the surfaces of the sleeve member.

As can be seen in FIG. 2, when the sleeve member 11, covered by the elastomer skin 12, is assembled on the sub-base so the rib 15 is received in the cooperating groove 16, flat headed screws 17 interconnecting these parts can be pulled together so the raw edges of the elastomer skin are clamped between the bottom 14 of the sleeve member and top of the sub-base. While the elastomer skin is bonded to the sleeve member, the above clamping arrangement prevents the edges from peeling away from the sleeve member when the elastomer is placed under tension during removal of the pattern P from a cured sand mold. The clamping action on the elastomer skin is not so severe that it will cause it to bulge from between the clamping surfaces.

With the pattern P assembled, its sub-base can be secured in mold table 20 with a tap bolt 21 in one of the several circular recesses 22 provided for this purpose, as the mold table normally accepts a plurality of such patterns.

In FIG. 1, it can be seen that the sub-base has a height equal to the depth of its recess 22 so the peripheral lip of the elastomer skin 12 extends across the joint between the sub-base and the table thereby providing a pattern protrusion from the table which is completely encased, with a seamless elastomer skin.

As mentioned above, the elastomer skin 12 is bonded to the sleeve member 11 and in order to maintain the desired dimensional tolerances in the final casting obtained from the resulting mold cavity, the thickness of this skin should be in the range of 0.010 to 0.050 inch, with the exception of the small backdraft areas where the thickness is somewhat greater. This thin skin has sufficient elasticity to allow the pattern to be extracted easily from the cured mold cavity while allowing the supporting sleeve member to support the skin in a manner adequate to preserve the desired tolerances; i 0.010 to 0.050 inch, depending on the diameter of the circular casting.

The circular or cylindrical sleeve member 11 which supports the thin skin 12 preferably has a slight outward taper in its inner circular wall 30 from its bottom 14 to its top rim 31. Its outer circular wall 32 has an appreciable inward taper from its bottom to the top rim, so the cross section of the sleeve member is somewhat triangular as can be seen in FIG. 2. These tapered surfaces along with the elasticity of the skin 12,. enables the pattern to be extracted easily from the cured sand mold cavity without damaging it.

The elastomer skin 12 is preferably formed of a silicon rubber, such as the type commercially available as Dow Corning Rubber No. 52, since it provides excellent heat resistant properties, good flexibility and can be easily bonded to the metal sleeve member 11. As illustrated in FIG. 4, the backdraft portion 13 of the pattern is formed completely of the elastomer since it must deform when the pattern is removed from the cured sand mold cavity. Normally this backdraft portion is relatively small in comparison to the complete pattern since a large, unsupported section of elastomer would not have sufficient rigidity to maintain desired dimensional tolerances when the sand is packed around the pattern. It is supported by the top rim 31 of the sleeve member and preferably tapers to a smaller cross section from the rim to its outer circular flaring periphery. Since the pattern is circular, this structure also tends to prevent the backdraft portion from deforming or distorting as the sand is packed around it.

With the pattern P mounted on the mold table as illustrated in FIG. 1, a sand mixture S is packed around it and others attached in the table. The sand mixture is of the curable kind, being either heat or gas cured so it will harden into a mold cavity M around each pattern on the table.

In FIG. 5, the separation of the pattern from the mold cavity M is illustrated. As the sand mold parts from the pattern P the elastomer skin on the walls 30 and 32 stretches, being pulled by the portion in the backdraft portion 13, which causes the skin to undergo a reduction in thickness which aids in separating the pattern from the mold cavity as the overall dimensions of the pattern decrease. At this time the backdraft portion elongates, reducing its cross section as it releases from the backdraft area, which allows this portion to also slip free without damaging the mold cavity. However, since the elastomer is bonded to the surfaces of the sleeve this action is limited.

Of course the sand has an abrasive action on the elastomer skin and therefore the sleeve member and its skin have a limited service life while the sub-base can be used almost indefinitely. If the skin is not bonded to the sleeve member is separates therefrom when the pattern is removed from the mold cavity and does not readily re-conform to the surfaces of the sleeve member. Thus, tolerances would vary far outside those desired in such a situation.

I claim:

1. An elastomer skinned pattern for forming circular one-piece molds having a backdraft portion comprismg:

a rigid sleeve member having a base portion and wall portions tapering from said base portion;

an elastomer skin contiguous to said wall portions and said base portion, said elastomer skin having an integral backdraft portion whereby the outer surface of the pattern can elastically deform to prevent damage to a cured mold formed therewith when the pattern is removed; and

a sub-base means attached to its base portion, said sub-base means operable to clamp the edges of the elastomer skin between said sub-base means and said base portion of the rigid sleeve member.

2. The pattern defined in claim 1 wherein the elastomer skin, exclusive of the backdraft portion, has a thickness of from 0.010 to 0.050 inch.

3. The pattern defined in claim 1 wherein the elastomer skin including its integral backdraft portion is formed of silicon rubber.

4. The pattern defined in claim 1 wherein the diameter of the sub-base means is approximately the same as the diameter of the rigid sleeve member whereby the elastomer skin will extend over the circular outer periphery of said sub-base means.

5. The pattern defined in claim 1 wherein the rigid sleeve member is a metallic sleeve and the elastomer skin is bonded to its wall portions and said base portion.

6. The pattern defined in claim 1 wherein both the wall portions of the rigid sleeve member are tapered from its base portion.

7. The pattern defined in claim 1 wherein the backdraft portion consists of a flange shaped portion of elastomer adjacent to the top of the rigid sleeve member.

8. The pattern defined in claim 4 including a mold table having an aperture for receiving the sub-base means whereby the elastomer skin on the base portion of the rigid sleeve member seats on the surface of said mold table whereby all surfaces of the pattern projecting into a cured mold cavity formed therewith will be elastomer coated. 

1. An elastomer skinned pattern for forming circular one-piece molds having a backdraft portion comprising: a rigid sleeve member having a base portion and wall portions tapering from said base portion; an elastomer skin contiguous to said wall portions and said base portion, said elastomer skin having an integral backdraft portion whereby the outer surface of the pattern can elastically deform to prevent damage to a cured mold formed therewith when the pattern is removed; and a sub-base means attached to its base portion, said sub-base means operable to clamp the edges of the elastomer skin between said sub-base means and said base portion of the rigid sleeve member.
 2. The pattern defined in claim 1 wherein the elastomer skin, exclusive of the backdraft portion, has a thickness of from 0.010 to 0.050 inch.
 3. The pattern defined in claim 1 wherein the elastomer skin including its integral backdraft portion is formed of silicon rubber.
 4. The pattern defined in claim 1 wherein the diameter of the sub-base means is approximately the same as the diameter of the rigid sleeve member whereby the elastomer skin will extend over the circular outer periphery of said sub-base means.
 5. The pattern defined in claim 1 wherein the rigid sleeve member is a metallic sleeve and the elastomer skin is bonded to its wall portions and said base portion.
 6. The pattern defined in claim 1 wherein both the wall portions of the rigid sleeve member are tapered from its base portion.
 7. The pattern defined in claim 1 wherein the backdraft portion consists of a flange shaped portion of elastomer adjacent to the top of the rigid sleeve member.
 8. The pattern defined in claim 4 including a mold table having an aperture for receiving the sub-base means whereby the elastomer skin on the base portion of the rigid sleeve member seats on the surface of said mold table whereby all surfaces of the pattern projecting into a cured mold cavity formed therewith will be elastomer coated. 